The present invention relates to flat displays and methods for their construction. More particularly, the present invention relates to a field emitter flat display having an inner vacuum space. The displays of this kind are often referred to as FEDs (Field Emitter Displays) and belong to the wider family of Flat Panel Displays (FPDs).
In detail, a FED contains a plurality of pointed microcathodes (microtips), which emit electrons, and a plurality of grid electrodes, placed at a very short distance from the cathodes, so as to generate a very high electric field. Between the cathodes and the phosphors is a vacuum space, which may be in certain cases some tens to some hundreds of micrometers (.mu.m) thick. The cathode may also be a diamond emitter. The vacuum in the vacuum space is usually kept under 10.sup.-5 millibar (mbar).
Sometimes the points of the microcathodes, the grid electrodes and the phosphors are aligned on a single flat surface, as described by Henry F. Gray, "Information Display" (3/93, page 11).
The patent document EP-A-0443865 describes a process for preparing a FED wherein a non-conducting substrate, for instance quartz, which supports the microcathodes and possibly the grid electrodes, in addition to possible auxiliary acceleration-anodes, is coated, in a part thereof free from cathodes and other electrodes, with a thin layer of an evaporable getter alloy based on barium, for instance BaAl.sub.4.
The thus obtained FEDs, however, present some disadvantages; in fact, getters of this kind require, to be operative, an activating heat-treatment (&gt;800.degree. C.) which may be usually carried out by means of radio frequencies, emitted by induction coils outside the FED. In cases where an evaporable getter material is employed, the heat-treatment should deposit a film of metal (for instance barium, one of the most commonly used evaporable getters) on well-defined and localized zones of the inner surface of the FED.
As barium is a good electrical conductor, its deposits, especially in a very small space as in the FEDs, may cause short circuits or electric breakdowns of the insulating surfaces; furthermore, such treatment may cause localized thermal shocks so as to seriously endanger the mechanical resistance of the FEDs.
Generally, the very small available space hinders the insertion of a getter having enough gas sorption capacity.
Some workers, in the past, have proposed to add to the displays an appendix or "tail" C, as shown in FIG. 1, intended to house a getter G without interfering with the thickness of the vacuum space between microtips MT and screen SCH. Such a technique, however, excessively increases the thickness, and therefore the volume, of the displays.
More recently, the application EP-A-572170 suggests to substitute the evaporable getter with other particular kinds of getter, for instance zirconium, which belong to the family of the non-evaporable getters (NEG), preferably present in large amount, such as, for example, microcathodes (microtips).
However, this suggestion is not free from negative consequences: the electronic emission of the sharp point of the microtips, if exposed to oxygenated gases, may be changed because of the production of zirconium oxide.
Another disadvantage is due to the difficulties which arise when the microtips are created, usually through a chemical etching of preformed layers. This technique leaves foreign materials within the microtips, which therefore lose most of their gettering capacity.